Atmospheric Pitting Corrosion of Al Alloys

Atmospheric corrosion involves open circuit exposure of metals to thin and sometimes concentrated electrolyte layers. Localized corrosion occur during atmospheric conditions as the result of the drying of a droplet of a salt solution on the surface as the temperatures increases or RH decreases, and the associated increase in chloride concentration. Under open circuit conditions, pitting initiation is accompanied by a sharp drop in the open circuit potential. This scenario is very different than the typical experimental conditions, which use bulk solutions and potential scanning.  Predictions of atmospheric pitting corrosion require an understanding of the controlling parameters under relevant conditions. We have previously studied this for stainless steel using a Scanning Kelvin Probe (SKP).   In this work we take similar approaches to study Al alloys.  Small droplets of MgCl₂ solution were placed on an Al-0.63Mg-0.28Si alloy and allowed to dry in a fixed RH environment while monitoring the Volta potential (which correlates to the corrosion potential) with SKP. Observations of corrosion phenomena vary with initial concentration of MgCl₂ and relative humidity, and include metastable pitting, stable pitting, and a unique filiform-like corrosion at the edge of the droplets. Of particular interest is the filiform-like corrosion; the mechanism and range of conditions under which it occurs is currently under further investigation.